Ann Stringer, director of human resources and organisational development at Northumbria Healthcare NHS Foundation Trust said: “We were shocked and saddened to hear the news of Heather’s death at the weekend and on behalf of the trust board we extend our deepest sympathies to Heather’s family, friends and all those colleagues who knew and worked with her. Heather was deeply committed to her role looking after patients in Northumberland and we will do all that we can to support her family and our staff at this incredibly difficult time.”

Friends and fellow competitors in the Mighty Deerstalker, which has 10-mile and five-mile races, reacted to the heartbreaking news on social media.

Debra Thorpe said: “Utterly heartbroken.”

Louise Robinson wrote: “In total shock. Bless her.”

Sharon Hetherington said: “I just feel sick. Can’t eat. Devastated.”

A nurse from Morpeth, Northumberland died during an endurance running race

About 2000 people are believed to have taken part in the endurance trail on Saturday, where runners tackled hills, mud and water hazards in the dark.

All the participants were warned in the race literature that they would have to wear head torches to be able to compete.

A spokesperson for race organisers Rat Race Adventure Sports said: “We are extremely saddened to confirm that a participant in our Mighty Deerstalker Event passed away yesterday. The 10 mile trail race was being held at Innerleithen, Tweeddale, Scottish Borders, in good weather conditions.

“The participant became unwell at 5.56pm, approximately one mile into the event, close to our event control station and at the location of one of our safety marshals. A member of our medical team was on the scene within three minutes as we implemented our response protocols.

“They were assisted by an off-duty paramedic until the arrival of the Scottish Ambulance Service. Tragically, we have since learned that the individual passed away on route to hospital. Rat Race Adventure Sports will give full assistance to the appropriate authorities.

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“Our heartfelt thoughts and condolences are with the family and friends of the deceased.”

A Police Scotland spokesperson said: “Police in the Scottish Borders responded to a report of a woman having taken unwell during a sporting event near to Traquair House, Innerleithen at 6.15pm on Saturday 11th March.

“The 48-year-old was treated by the Scottish Ambulance Service and was taken to Borders General Hospital. However, she passed away while en route.

“Inquiries are continuing, however there appear to be no suspicious circumstances surrounding this death and a report will be sent to the Procurator Fiscal.”